White lines on coralline algae

[SuncrestReef]

Now that my rocks are almost completely covered in purple coralline algae, I've started to notice some areas with distinct white lines across the coralline. Could these be some sort of snail trail, or perhaps some other phenomenon? They are definitely not cracks in the rocks. These markings or patterns were not there before the coralline, and even for a while the coralline was solid purple in these areas. The white lines appear later.

I'm not worried about it, just very curious what might cause this.

 

[Coralreefer1]

The only thing I have is something is making a trail as it eats the coralline, much like a Sea Urchin might do.